Saturday, June 26, 2010

Q Phil Speer, KF4VSK, asks, “I recently bought a 6-meter FM mobile transceiver that I often use at home with a simple dipole about 18 feet off the ground. When working simplex it seems that my maximum range is only about 20 miles. Shouldn’t I be able to do better than this on 6 meters?”

A Yes, you can do better, but there are a number of factors at play. The principal factor is terrain. If your home is surrounded by hills, mountains or other obstacles, your simplex range on 6 meters will be limited. In addition, you’re not using a gain antenna (such as a Yagi) that would focus your power and provide greater range.

Despite all this, you should be able to work hundreds or even thousands of miles when the band is open. Spring and summer are among the best times for sporadic E band openings. Since your radio is FM only, I’d suggest that you leave it on 52.525 MHz, the national 6-meter FM simplex calling frequency. Just turn up the volume and close the squelch. If there is a band opening, you’re likely to hear FM simplex activity on this frequency. In addition, the next year or two will likely yield some F-layer skip, which can carry your signal over huge distances. Watch the propagation forecasts and keep your ear to your radio! See Feedback in July 1999 QST.From QST June 1999

A The traps in a trap dipole are parallel-resonant tuned circuits that “trap” an RF signal to prevent it from passing beyond a specific point on a conductor (a wire, a metal tube, etc). At some other frequencies, however, the traps no longer act like “traps” and instead allow the RF to pass.

An antenna trap is designed for a particular operating frequency, and there may be several traps in the overall system, each designed for a specific frequency. Therefore, a 40- through 15-meter trap antenna, like the trap dipole shown in Figure 3, might contain traps for 20 and 15 meters. When you’re operating on 15 meters, the 15-meter traps effectively “shorten” the antenna by blocking the RF from traveling beyond them. If you switch to 20 meters, the 15-meter traps suddenly become transparent to the 20-meter RF, effectively “lengthening” the antenna. The 20-meter traps, however, are a kind of impedance roadblock to RF, keeping the signal from traveling farther. On 40 meters, all of the traps are “absorbed” into the system to become part of the overall 40-meter dipole antenna.

Because of the loading effect of the traps, the 40-meter portion of the antenna will be somewhat shorter than a full-sized 40-meter dipole without traps. The effective bandwidth on each band will be narrower than that of a standard dipole, too.

A trap-style antenna is not quite as efficient as a full-size dipole, but if the traps are well designed, the losses are not significant. Most hams consider the losses a fair trade-off for the convenience of having an antenna that presents a 50-Ω match to coax on several bands.

Figure 3—A typical 3-band trap dipole antenna. The traps block RF at a specific frequency, or allow it to pass. From an electrical standpoint, this effectively lengthens or shortens the antenna.

Q There has been some discussion lately about changing the name of the ARRL. As a new member I’m curious: What does the “Relay” in “American Radio Relay League” mean?

A In the early days of the League, hams (and everyone else) used frequencies at or below 200 meters (roughly 1500 kHz). The frequencies above 200 meters were considered useless, but hams were to disprove that quaint notion within another decade or so. Because of the limits of equipment and propagation at 200 meters and below, hams passed messages over great distances by relaying them from one station to another. This was one of the chief activities of amateurs in those days and it helped solidify Amateur Radio as a viable public-service network. So, it’s understandable that the founders of our organization would choose a name that reflected the nature of Amateur Radio at that time. If you want to read more about the early days of our hobby, I highly recommend 200 Meters & Down by Clinton DeSoto, W1CBD. See the ARRL Bookcase in this issue, or order on line at http://www.arrl.org/catalog/.

Q I’d like to homebrew a 5-V, 1-A dc power supply that Ican use with a frequency counter as well as some other projects. Can you suggestsomething that’s compact, inexpensive and uses easy-to-find components?

A If you’re going to challenge the Doctor, you have to do better than that! But seriously, take a look at the diagram in Figure 2. This little 5-V supply should give you decent regulation. It meets your cheap-and-easy test, too.

Figure 2—This little 5-V power supply is simple to build and should supply up to 1 A of current. Make sure you put a heat sink on U2, the voltage regulator. RadioShack part numbers are shown below.

Q Ron Vlach, WA0QMP, asks, “When I contact a DX station and he tells me to ‘QSL VIA CBA.’ What does that mean?”

A “CBA” means Callbook Address. He is saying that you can send your QSL card to him directly by looking up his postal address. The Callbook is the Radio Amateur’s Callbook, which used to be a telephone-directory style publication, but is now available on CD-ROM only. The term is rather generic, though. (Like saying, “Please give me a Kleenex” when you really just want any brand of tissue.) When DX stations tell you to send cards to their “CBA” addresses, they often mean the addresses shown in any number of DX address databases, not necessarily the Callbook alone.

Q When my shack computer is booting up a quick message flashes by concerning my hard drive. It is something about my drive being SMART compatible. What does this mean?

A SMART stands for Self-Monitoring, Analysis and Reporting Technology. It is a feature of the hard drive that allows it to give advance warning of certain types of failures. Think of it as the hard drive reporting on the state of its health. The SMART concept seems terrific, until you realize that the drive manufacturers set the SMART parameters. That is, they decide how the relative health of the hard drive will be judged and reported. It isn’t in their best interests to have hard drives calling in sick, so to speak, at the drop of a hat. The Doctor has seen some very sick hard drives reporting 100% health, so the value of SMART is questionable at best. If your BIOS supports SMART you can usually enable it in your SETUP, but I wouldn’t expect too much. A religious devotion to backing up your drive on a regular basis is still the best insurance. After all, there are only two types of hard drives—those that have failed and those that haven’t . . . yet.

Q This is a follow-up to my question about a homebrew vertical in your column in November 1998 QST. Following your advice, I added a temporary radial to the six I already had and found the change in signal strength was so slight that I didn’t bother to add any more radials. Then, a few months later in another magazine, I saw a column by a writer who bills himself as the “Kaped Krusader.” He labeled your advice as “more antenna hogwash.” Who am I to believe?

A Old Doc saw that column too and wondered aloud: “Where has civil discourse gone?” Doc’s only human, though, and there are times when he is sorely tempted to put on his own cape and reinvent himself as the “Caped Curmudgeon.” But then civility and sensibility returns to Doc’s placid soul and he takes a deep breath and vows to remain positive, practical and helpful.

You wanted to gauge how much effort it would take to improve your vertical antenna system by adding radials. Doc’s previous answer was a thoroughly practical one. Adding one radial would be unlikely to produce much of an improvement. Even the Kaped Krusader seemed to agree with that, although he went on to say that adding a huge number of radials would improve the situation further. After you sort through all the Kaped Krusader’s tables and bombastic verbiage, adding 107 more radials will theoretically improve your signal by about 3 dB, one-half of an S unit.

The Kaped Krusader has stated many times before in print that 2 dB is barely detectable at the other guy’s receiver—and Doc agrees with this. So, you will have to decide for yourself whether an extra 3 dB is worth going through all the work of digging up your lawn to put down more than 100 radials. Yes, you will be beaten out more often in the big DX pileups without that extra 3 dB. But even if your vertical were theoretically 100% perfect, do you have any illusions that you’re going to beat out stations with big Yagis on tall towers?

Doc’s choice? I’d spend the time enjoying myself, making friends and working people on the air—and being civilized about it.

Q Kent Taylor, K4AHU, asks, “I have not been operating for several years and I am looking forward to getting back on the air. I have moved to North Florida and I would like to make regular contacts with friends 40 to 60 miles away. Would 10 meters be adequate for this purpose if I was running a beam and 100 W?”

A Ten meters is probably not the best band for your application. Depending on what the other stations are using for rigs and antennas, contact is probably possible, but the signals may be weak. You’d have better luck using 40 meters during the daytime and 80 or 160 meters at night.

QI’m trying to become active in slow-scan television (SSTV) with software that sends and receives images using my computer’s sound card. I can receive images just fine, but I’ve been told that my transmitted images are very distorted. From the way my transmitted tones sound, I suspect that RF is getting into my sound card. In fact, if I reduce power substantially, the distortion disappears. I’ve placed ferrite cores on all of the leads going in and out of my PC, as well as on the cable going from the sound card output to the auxiliary audio input on my transceiver. They helped, but they didn’t suppress the RFI completely. Can you suggest something else I could try?

A My suggestion would be to try to isolate the sound card audio output line from the transceiver. You can do this by simply adding a 1:1 isolation transformer in the line (Figure 1). You may still need to use the ferrites, however, to get full suppression.

Figure 1—Adding a 1:1 isolation transformer between the sound card output and your transceiver may help alleviate some RFI woes. The transformer shown in this illustration is a RadioShack 273-1374.

Q I just installed a Creative Labs “SoundBlaster” AWE64 sound card in my shack PC to replace my old SoundBlaster 16. My kids use the computer for games and homework and they wanted the enhanced sound capability. I wanted to try SSTV and the new PSK31 HF digital mode using the sound card as well. The problem is that Windows 95 keeps telling me that the AWE64 is not working, even though it detects it, and it insists that my old SoundBlaster 16 is still present! I’m confused.

A So is your computer! It sounds like you need to do a clean sweep of the Windows 95 SoundBlaster driver files, purging the old SoundBlaster 16 drivers and loading the new AWE64 drivers.

Grab the file SBW9XUP.EXE on the Web from http:// www.support.soundblaster.com/. This will give you a complete set of the very latest AWE64 drivers.

Q Bob Abbey, WN4J, asks, “I would like to begin conducting code practice sessions with local hams on 2-meter FM. Is this legal? Would it be a prohibited form of ‘broadcasting’”?

A There is nothing to stop you from transmitting code practice on 2-meter FM. In fact, W1AW does just that on 147.555 MHz. This is permitted by Section 97.111(b)(5). Concerning the “broadcasting” issue, this term is defined in Section 97.3(a)(10) as “Transmissions intended for reception by the general public, either direct or relayed.” All amateur transmissions must be intended to be received only by other amateurs. Nothing prevents a nonham from overhearing an amateur codepractice session—from a practical standpoint, we hope they do— but that sort of reception is unintentional.

Q Gerry Miller, AA2ZJ, asks, “I recently stumbled on what I believe are beacon signals from about 200 to 400 kHz. They identify in CW, sending their call signs over and over. Some of the call signs I’ve copied include CAT, PPK, UR and NEL. Can you solve this mystery for me?

A The signals you’ve received are indeed beacons. Specifically, they are nondirectional navigational beacons, many of which are maintained by the Federal Aviation Administration and the US Navy, among others. Most of the beacons identify themselves with two or three-letter call signs sent in Morse. These beacons can be heard over great distances when conditions are right.

There are radio hobbyists who embrace the challenge of “DXing” these beacons and even obtaining QSLs to confirm reception. One excellent tool for identifying the beacons can be found on the Web at http://www.airnav.com/navaids/. At this site you can plug in the call sign and see the location and other information about the beacon.

For example, you mentioned hearing NEL. According to the database, that call sign belongs to a Navy beacon located in Lakehurst, New Jersey.

Thursday, June 24, 2010

Q I live in an area where outdoor antennas and towers are restricted. I’ve noticed, however, that several of my neighbors have flagpoles; apparently they are acceptable. Is there any way to design a “stealth” antenna using a flagpole?

A Certainly! Flagpole stealth antennas are ham traditions that go back many years. Plastic or fiberglass flag poles make fine supports for vertical antennas that consist of little more than a quarter-wavelength of #10 wire snaked through the hollow center (see Figure 2). You can put a quarter-wavelength vertical antenna for 20 meters in a flagpole that’s only about 17 feet tall and feed it with 50-Ω coaxial cable buried in the lawn. And while you are digging, bury as many insulated copper radial wires as you can. Make the radials as long as possible, but don’t worry about their specific lengths. Quantity is what counts most.

I know of one ham who took a particularly clever approach with his flagpole vertical. He buried a remote-controlled antenna tuner in a weatherproof enclosure at the base of his 35-foot flagpole. As a result, he is able to load the antenna on 40 through 10 meters at the push of a button! Just more proof of the old adage, “Where there is a will, there is a way.”

Q When looking at the contest results in QST I often see a reference to “check logs.” What are check logs?

A Check logs are logs that, for various reasons, are not eligible for entry into the contest. A log that is submitted past the entry deadline, or one that arrives on time but can’t be used because of uncorrectable format problems, may be listed as a check log. A log might also be designated as a check log if there were discrepancies in the contest exchanges being sent, or if numerous scoring errors are discovered. By designating a log as a check log, it is listed in the results so that people know that station did submit a log to the Contest Branch. At times, participants may also submit logs for log-checking purposes only rather than as a competitive entry.

Don’t confuse check logs with logs used for crosschecking purposes by contest officials. With improved software and computer techniques, all logs that are submitted to Headquarters in electronic format are now being crosschecked against one another to ensure more accurate and complete contest score reporting.

Q Mike Usas, N8KXI, asks, “Which is the proper way to describe the 75/80-meter band? I’ve heard it called 75 or 80 meters by both old and new hams. Is it 75, 80 or both?”

A You’ve asked a good question. The answer falls into that gray area of Amateur Radio slang. For those who may be new to the hobby, the idea of using 75 or 80 meters to refer to the same band can be confusing.

Technically speaking, a wavelength of 80 meters corresponds to a frequency of 3750 kHz. A wavelength of 75 meters corresponds to 4000 kHz. When hams kick these numbers around in conversation they are usually referring to either the phone or CW/data segments of the band. When they say “75 meters” they are talking about everything from 4000 down to 3750 kHz—the phone portion of the band. When they mention “80 meters,” they’re usually discussing the band segment from 3750 kHz down—CW/data.

Of course, like anything else in common language, the terms are often interchanged, depending on who is speaking! For example, many hams simply say “80 meters” when they mean anywhere in the band.From QST May 1999

Q Larry Amann, K5TQN, asks, “I’d like to feed a dipole above my roof using my antenna tuner and a balanced feed line. However, my roof is metal and the feed line would have to rest on it for a considerable distance. I know that you need to keep open-wire feeders away from metal, but this is not possible in my case. Could I make my own shielded balanced line by using two lengths of coax in parallel?”
A The idea of using two parallel lengths of coax to create a kind of “shielded” balanced feed line has a long history. A number of hams have used this approach in situations where they needed to pass a balanced feed line over an expanse of metal, such as a metal roof. The technique involves placing two coaxial cables in parallel (using tape or cable ties to keep them firmly together along the entire length), shorting the shield braids of both coaxial cables at both ends, and grounding the “shack side” of the braids at your antenna tuner (see Figure 1). The inner conductors of the coax then become your “shielded” parallel feed lines.

The primary drawback with this technique is that you will not enjoy the same degree of low-loss performance as you would with standard twin-lead or ladder line. On the other hand, it should allow you to maintain a balanced line all the way to your antenna, despite your metal roof.

Figure 1—You can make a shielded balanced feed line out of two pieces of coaxial cable. It won’t have the same low-loss characteristics of open-wire or ladder line, but it is a workable alternative when the feed line must pass over a large metal surface.

Q Kevin Kalil, KF4ZQK, asks, “I purchased an H-T that was supplied with a screw-on rubber duck antenna. I also own a 1/4-wavelength telescoping whip antenna that I’d like to use, but it has a male BNC connector. Is there some sort of adapter that will allow me to use this antenna with my H-T?”

AThe type of connector your H-T uses is known as an SMA. In this case, it is a female SMA. To use your telescoping whip all you need is a male-SMA-to-female-BNC adapter. These are commonly available from several QST advertisers.

Wednesday, June 23, 2010

Q I enjoy 6 meters, even when the band isn’t open. I’ve found that it is a terrific band for “local” communications up to a few hundred miles away. Some of the local signals, however, have an odd fluttering characteristic. What causes this? Does the fact that I live near an airport have anything to do with it?

A Take a look at Figure 2. The energy traveling directly between the horizontally polarized transmitting station antenna and receiving station is attenuated to about the same degree as in free space. But unless the antennas are very high or quite close together, an appreciable portion of the transmitted energy is reflected from the ground as well as from buildings and towers. These two signals combine at your antenna, and that’s where things get particularly interesting.

When the signal strikes another surface, its phase is reversed. If the distances traveled by both signals were exactly the same, and if the reflection phase reversal was exactly 180°, the signals would arrive out of phase with each other and cancel completely. This never happens in the real world or you would hear nothing at all! Instead, the reflected signal travels a little farther. Combine this with the less-than-180° phase reversal and you have partial cancellation at the antenna, not total. Your statement about living near an airport provides an important clue. Signals bouncing off aircraft can arrive at your receiver with rapidly varying phase and amplitude, causing considerable flutter.

Figure 2—Part of the signal energy takes the direct path to the antenna, but another portion arrives as a wave reflected from the ground or other objects. There is a phase reversal with each reflection, and the distance the wave travels is greater as well. The signals combine at the antenna, adding and subtracting from each other.

Q Daryl Pate, KC5SLQ, asks, “I recently began using an Alinco DX-70 HF transceiver with an antenna tuner and a multiband wire dipole antenna (using a coaxial feed line connected directly to the antenna—no balun). For a while, everything was fine. Now, however, I suddenly find that the SWR remains extremely high on all bands. I inspected the antenna and the feed line and they appear to be okay. Do you have any suggestions?”

Figure 1—A volt-ohm meter provides a quick test for possible feed line shorts (A). Test the function of the tuner and feed line by substituting a dummy load for the antenna (B).A Obviously something has changed in your tuner and antenna system. Try the easy steps first (see Figure 1). Get a volt-ohm meter (VOM), disconnect the antenna coax from the tuner and measure the resistance between the center conductor and the connector shell. It should be infinite. If the VOM reads zero, you have a short somewhere in the feed line or at the antenna. Inspect the antenna again. Look for loose or broken connections between the coils or traps.

If you can get your hands on a dummy load, disconnect the coax at the antenna and substitute the dummy. A dummy load is just a resistor (or several resistors) in a box or can. It acts like an antenna without radiating much RF. Your transceiver should see about a 1:1 SWR on all bands and your tuner should be able to “match” this easily. If your tuner and transceiver behave properly when the dummy is connected, you’ve just eliminated them from your list of possible suspects.

But if the antenna tuner still doesn’t work with the dummy load, it’s time to pop the cover and do a visual inspection. Rotate both tuning capacitors. If you hear a mild scraping or feel the plates rubbing each other at any points in their rotation, you must attempt to reposition the plates so that they don’t touch at any point. Look for errant blobs of solder that could be shorting a coil or capacitor. Gently tug on the wires to make sure they are firmly soldered in place.

Tuesday, June 22, 2010

Q What are loop Yagi antennas and why can’t you use them at HF frequencies?

A No one said that you couldn’t use a loop Yagi on the HF bands, but you’d really be pushing the envelope of practicality!

Loop Yagis are members of the quad family since each element is a closed loop of approximately one wavelength at the operating frequency. Line up a number of loops on the same boom and you create a fairly high-gain antenna. The loop Yagis in Figure 3 can develop about 20 dBi gain (each) at 1296 MHz, but the individual loops are only a few inches in diameter. That’s a substantial amount of gain in a relatively compact space.

But imagine supporting a similar collection of one-wavelength loops at, say, 14.2 MHz. The antenna would be monstrous! If you’re a lover of rotatable loop-style antennas at HF frequencies, a traditional quad design is far more practical.

Q Evan Scarborough, N1ZHD, asks, “Is it possible to work OSCAR 27, the FM repeater satellite, using 1/2 W of power on the 2-meter uplink?”

A For those unfamiliar with the bird, OSCAR 27 is a loworbiting Microsat that presently functions as an FM repeater. It listens on 145.850 MHz and repeats on 436.800 MHz. The satellite is available during daylight passes only.

You could probably work OSCAR 27 with 1/2 W, but you would need a beam antenna—possibly a long-boom model with a number of elements—to focus your power. Of course, the higher the gain of the antenna, the narrower the radiation pattern. This means that aiming becomes much more critical.

The problem with running low power while trying to work OSCAR 27 is that this satellite can only accommodate one signal at a time. Whenever it speeds over the US, many operators attempt to use it simultaneously. With the FM “capture effect,” only the strongest signal “wins.”

Q Ray, WX3A, asks, “I have two towers of different heights: one with a 2-meter beam above an HF tribander, and the other with a 440-MHz Yagi above a 6-meter-beam. I’d like to determine what my ERP would be at:

Frequency Power Antenna Height

Is there a simple way to do this?”

AEffective Radiated Power (ERP) is actually fairly easy to calculate, as long as you know the gain figures of your antennas and can live with a few (supportable) approximations. ERP is related to the gain of a dipole in free space. To calculate ERP, you simply multiply the power at the antenna by the gain of the antenna, referenced to a dipole. The actual gain of an antenna over ground is about 4 dB higher than the “published” gain of the antenna, due to ground reflections.

Let’s start with the HF calculation. To determine the RF power radiated by the antenna, we must reduce the transmitter power in the shack by the amount of loss in the feed line, then boost that by the gain of the antenna. Assume that your HF tribander has 6.5 dBi of gain at 29 MHz in free space, with a transmission-line loss of 1.5 dB. This is about the loss that can be expected for 100 feet of RG-213 coax at this frequency for a low SWR. This is a net gain of 6.5 – 1.5 = 4.0 dBi. Next, you must reference the net gain to a dipole (dBd), not an isotropic radiator (dBi), so subtract 2.15 dB. This gives a net gain of 1.85 dBd, but this is in so-called “free space.”

You’re obviously going to be placing the antenna over ground rather than in theoretical free space, so there will be some “ground gain,” at a peak angle determined by how high the antenna is located over ground. The exact angle isn’t important for this discussion, since we’re looking for the peak radiated power. EPA documentation assumes that reflection from typical ground will give another 4 dB over the net gain in free space. So, now we’ve got 1.85 + 4.0 = 5.85 dBd, due to ground gain. Convert this to a numerical ratio by raising 10 to the power of 5.85 divided by 10 = 105.85/10 = 3.85. This is the number we now use to multiply the transmitter output power to give the ERP: 3.85 × 1500 W = 5775 W = 5.78 kW.

For VHF, let’s assume that you have a high-gain Yagi with 15 dBi gain in free space on 432 MHz. Here, the feed line loss will be more significant than on HF. Assume that you are using 100 feet of high-quality Belden 9913 coax, which will have about 2.8 dB of loss on 70 cm. The net gain, in dBd over ground is: 15.0 – 2.8 + 4.0 - 2.15 = 14.05 dBd, which is a numeric ratio of 25.41. Thus the ERP at 432 MHz would be 1.5 × 25.41 = 38.11 kW, a very substantial number indeed.

Now, there is another term in use, EIRP, or Equivalent Isotropically Radiated Power. This term is not referenced to a reference dipole in free space but to an isotropic radiator in free space. You can get EIRP by multiplying ERP by 1.64, the equivalent of 2.15 dB.

Q Paul Taylor, WB2GIN, asks, “While repairing a rig I installed a replacement transistor, but now the circuit oscillates on a frequency near TV channel 2! I have read that tiny ferrite beads can be employed to correct this problem. How are they used?

A We occasionally see this on replacement transistors—they can have a higher cutoff frequency than the original so they may oscillate. This happens because parasitic resonances in the circuit that would not have mattered with the original device now become a problem because the replacement transistor is able to amplify at the higher frequency.

Most of the time, a ferrite bead and/or a 10-Ω resistor in the base circuit usually tames things down. If you have a few beads, you may need to experiment, perhaps with two beads on the base lead. Try different material (for VHF, I would use #43 or equivalent material) and perhaps even adding a bead to the emitter and/or collector if the base bead didn’t do it. Unfortunately, the exact circuit configuration often determines where suppression is needed. If the resonance is in the base circuit, base beading will be most effective. If it is in the collector circuit, a bead on the collector would be most useful.

A Common-mode currents on antenna feed lines are often the culprits when directional patterns are distorted or SWR readings become unpredictable. The solution is a balun—a contraction of the words balanced to unbalanced. Baluns eliminate these common- mode currents while making the transition between an unbalanced feed line (such as coaxial cable) and a balanced load (such as an antenna).

Figure 2—The W2DU balun consists of ferrite cores over a length of coax.

There are many types of balun designs, but Walt Maxwell, W2DU, developed the one you’re asking about. It is a choke-type balun that consists of a number of small ferrite cores placed directly over a section of coaxial cable where it is connected to the antenna. The W2DU balun in Figure 2 is a low-power version using 50 Amidon FB-73-2401 cores slipped over a 1-foot length of RG-58 coax. Twelve Amidon FB-77-1024 cores on RG-8 or RG-213 will do the same job. The 70-series cores are best for HF work. Type 43 or 61 is best for VHF. The W2DU balun is very effective and, best of all, very easy to make.

Q NT9N asks, “I’d like to install a 40-W, 2-meter mobile transceiver in my car. Why can’t I tap the +12 volts from the fuse block inside the vehicle instead of going directly to the car battery? Getting a wire through the firewall seems difficult and I don’t want to drill holes. Any ideas?”

A Even though you could indeed power a 40-W rig from the fuse block, there are still benefits to connecting to the car battery directly. By obtaining your power at the battery you reduce the possibility of causing voltage drops at various points in the car’s electrical system—drops that might cause sensitive electronics to malfunction. In addition, the low internal resistance of the car battery may act like a filter capacitor and short any stray RF to ground instead of allowing it to propagate through the rest of the wiring.

Getting through the firewall to the battery isn’t as hard as it seems. You should be able to find a little rubber plug about the size of a bottle cap (or larger) on the firewall that can be removed or sliced open to allow your power cables to pass. You may also be able to snake your power cable through a grommet used by an existing wire harness. Just be careful not to damage the other wires.

There is much more information available. Mobile installation subjects are covered in the ARRL Handbook and the ARRL RFI Book.

A BNC is a “quick connect/disconnect” coaxial cable connector (see Figure 1). Unlike common PL-259 or male N connectors that require several rotations of their shells to secure them, a BNC latches with a single twist. According to Press “The Wireman” Jones, N8UG, “BNC” is an abbreviation for “Bayonet Neill Concelman.” The word “bayonet” refers to the connector’s style and Paul Neill and Carl Concelman were Bell Labs engineers who developed it. BNC connectors can be used well into the microwave range and can tolerate as much as 500 V peak-to-peak.

Q I’m about to install a homebrew vertical antenna and a system of buried wire radials. My only question concerns the radials. What sort of wire should I use? I would imagine that some wires would corrode in soil faster than others.

Divining an answer to this question is trickier than it seems. Being more of a general practitioner, the Doctor opted to consult with three prominent antenna specialists: Dean Straw, N6BV, Roy Lewallen, W7EL, and Tom Rauch, W8JI. The following answer summarizes their comments.

A Noninsulated copper wire can be expected to last several years in just about any kind of soil. Insulated copper wire is even better. Copper-clad steel wire should be avoided, however, because it has a relatively short life. And stay away from aluminum wire; it will turn to powder in a year or less!

The problem isn’t just the pH of the soil, but what the radial eventually connects to via the shield at the station end. It’s quite easy, even in what appears to be “favorable soil,” to have an electric potential that erodes the radials since there is a complex dc path that involves everything connected to the radial system. The only safe solution is to use copper wire—#16 or larger—bare or otherwise. Some bare copper radial systems buried at broadcast sites in the 20s have been uncovered and found to be virtually perfect! It’s hard to go wrong with copper.

Q John Stewart, W3CID, asks, “I use a 40/80 meter vertical antenna (a Butternut HF2V) to work the higher bands with the help of an antenna tuner. Ignoring the effects of line losses because of the (presumably) lower SWR at the feed point, would I gain any efficiency by switching to a similarly sized vertical designed for multiband operation? Would the performance of the HF2V on other bands be improved if I added shorter radials cut for the higher frequencies?”

A The crux of the problem with your present system is that you cannot ignore the effect of line losses when you try to use the HF2V on higher frequencies, where it is not resonant. In other words, the SWR on the higher bands is not low. While your antenna tuner in the shack is able to provide a 50-Ω load to your transceiver, there is probably quite a bit of loss in the transmission line between the tuner and the antenna. Remember: The 1:1 SWR you see on your antenna tuner’s meter is only present between the tuner and the radio. The higher SWR between the tuner and the antenna, and the resulting loss in the feed line, remains! In addition, there may be considerable loss in the tuner itself since it may be encountering impedances that are difficult to match efficiently.

Let me illustrate, using a model of a simple quarter-wavelength long vertical for 40 meters. I’m going to assume that the ground plane is perfect, so that we have a baseline from which to compare. At 7.1 MHz the feed-point impedance is the theoretical value of 36 Ω and the SWR at the feed point is 1.39:1 for the 50-Ω line. (I’ll also assume that the feed line consists of 100 feet of RG-213 coax.) The total loss in this coax at 7.1 MHz is 0.566 dB, computed using the program TLA bundled with the 18th edition of The ARRL Antenna Book. The coax loss is essentially the matched-line inherent loss if the cable were working directly into a 50-Ω load. There is very little additional loss due to the small SWR at the load.

Now, this very same vertical at 14.1 MHz would be close to a half wavelength long and the feed-point impedance would be very high. The EZNEC program by W7EL computes it to be 814 + j 119 Ω. At this impedance the SWR on the RG-213 would be an impressive 52:1, and the loss in the cable would now be 7.5 dB! The loss in a typical antenna tuner feeding the input of this 100- foot length of coax would be on the order of an additional 0.35 dB. Feeding 1500 W into the tuner would result in only about 247 W radiated by the antenna! Something is going to get hot, mainly the coax.

At 21.1 MHz the situation would be somewhat better since the 40-meter vertical is three quarter wavelengths long and the feedpoint impedance would be 63.4 –j 58.8 Ω according to EZNEC. This is an SWR of only 2.77:1 and the total loss in 100 feet of RG-8 would amount to only 1.47 dB, according to TLA. A typical tuner would lose only a negligible amount more, again because the impedance to be matched is reasonable. In this scenario, the total power delivered to the antenna for 1500 W input is 1057 W. That’s a lot better than on 20 meters.

Unfortunately, another little problem rears its head at this point. Because the electrical length of the antenna is long at 21.1 MHz, the radiation pattern has developed lobes pointing up in the air. You’re heating the clouds rather than aiming for lower angles that are most useful for DXing on the higher frequencies. This problem will get only worse on 12 and 10 meters where the electrical length is even greater.

So, you can see that having a multiband antenna that is quarterwavelength
resonant in the bands you desire would provide far better performance than trying to force-feed your existing dualband antenna by using an antenna tuner in the shack. Of course, you could move your antenna tuner to the base of your Butternut. In this position most of the loss would be in the tuner only. But unless you installed an automatic tuner at the base of the antenna, it would be very inconvenient to use. (See “One Stealthy Wire” by Steve Ford, WB8IMY, in the October 1998 QST.) And you would still be presented with the problem of energy wasting higher angle lobes due to the electrical length of the antenna.

My recommendation would be to switch to a multiband vertical. Butternut and other manufacturers make such antennas.

Q Jim Brown, K5JAZ, asks, “I have a signal generator with a meter calibrated 0-10 μV. I am searching for the formulas and printed tables to convert μV to μW and dBm. Can you help?”

A Signal generators are usually calibrated to deliver their rated voltage into a specific value of resistive load. Most modern test equipment is calibrated in a 50-Ω system; equipment designed to test televisions and television systems is calibrated for a 75-Ω load. Some equipment, usually audio or telephone equipment, is calibrated into 600 Ω.

Let’s assume that your generator is calibrated for 50 Ω. What this means is that it will deliver what is indicated on the outputlevel control and multiplier if it is operated into a 50-Ω load.

With this assumption, you can use Ohms Law to convert from μV to μW. The formula P = (E2)/R works if the units are volts, watts and ohms. If you wish, you can convert the μV to volts, obtain the power in watts, then multiply that result by 1,000,000 to convert the result to μW. In this case, we are assuming R to be 50 Ω.

To wrap that all into one formula, you can do the conversions
all at once, using μV and ohms and obtaining a result in μW by:

P(μW) = (E(μV)2)/R)/(1,000,000)
and, if you know R to be 50 Ω, you can use:P(μW) = (E(μV)2)/(50,000,000)

The term dBm means decibels related to a milliwatt, so, you can either convert the value in microwatts to milliwatts by dividing it by 1000, then use the formula:

dBm = 10log10(P(mW)) where P = the actual power in milliwatts, or, to do it all in one motion, you can use the formula:

Q Dan Marriott, VE7CTN, asks, “I am looking, like most hams, for points of comparison when I read QST Product Reviews. And, while I think I understand what some of the tests mean, their labels or abbreviations are not always intuitive.For example, when evaluating receive performance I see references to MDS, IMD, IMD dynamic range, blocking dynamic range, third-order products and third-order intercept points. Can you clarify?”
A These terms, and many others, are described in great detail in the ARRL Handbook’s Test Procedures and Projects chapter. A collection of QST articles that explain the Product Review process and tests can be downloaded from the ARRL Web page at http://www.arrl.org/tis/info/bestrig.html. Here is a very brief summary that will get you started.

The MDS (minimum discernible signal, or “noise floor”) is a measure of receiver sensitivity. It describes the amount of receiver input noise. A receiver should be able to just detect a real signal at the level of the noise floor, thus the term, minimum discernible signal. MDS is usually expressed in dBm—decibels relative to a milliwatt. A typical receiver might have an MDS of –135 dBm, or 135 dB less than a milliwatt. That is an extremely small amount of signal power. The MDS numbers give you a reasonable idea of a receiver’s sensitivity. Useful SSB sensitivity usually falls in a range of about –120 dBm to –135 dBm.

Intermodulation distortion (IMD), whether transmit or receive, is the mixing of two frequencies to produce additional frequencies. Recall that the chief property of a mixer is to produce sum and difference frequencies by mixing signals from the input with those from the local oscillator. Living in an imperfect world as we do, electronic circuits are not perfectly linear so additional mixing takes place between these original frequencies and the ones that are intended to appear.

Assume that your original frequencies are F1 and F2 with the difference between them being N Hz. The combination of all the above mentioned mixing creates undesirable signals at 2F1 – F2 and 2F2 – F1 (among others—these are just the ones that usually affect transmitter or receiver performance). These end up appearing at N Hz above and below F1 and F2. They are known as third order products. There are other signals that appear as well (on a transmit IMD graph, you typically see third, fifth, seventh and ninth-order products or more), but these are not as high in amplitude as the third-order ones.

How does this affect real-world operating? On receive, if your radio is tuned to 14.020 MHz and two strong signals appear at 14.040 and 14.060 MHz, assuming nothing else is on the band, you will get a false signal appearing on 14.020 where you are tuned. Under test conditions, the two off-channel signals are identical in strength, at a level that gives an IMD response equal to the MDS level. The difference in strength between the on-channel signal “ghost” and the two off-channel signals is the IMD dynamic range, expressed in dB.

This test indicates the general intermodulation behavior of a receiver. If, for instance, you were slugging it out with the DX crowd trying to work a rare one, you would have signals of many different strengths appearing at many different frequencies inside and outside the passband of your receiver’s filters. When the signals are strong enough, intermixing will produce false signals within the receiver’s passband. The higher the IMD dynamic range of your receiver, the weaker (and less annoying!) these false signals will be. Typical receivers will have an IMD dynamic range of from 80 to about 105 dB.

Blocking dynamic range is basically a measure of how strong an off-channel signal must be to produce either an increase in noise in the receiver passband or a decrease in receiver gain, otherwise known as “desense.” In ARRL Lab tests we use a signal that is 20 kHz away from where the receiver is tuned. A signal within the normal operating amplitude of the receiver is added on frequency and then the level of the off-channel signal is increased until the on-channel signal decreases by 1 dB (blocking is occurring) or the output increases by 1 dB because of receiver noise. (In this case, the measurement is reported as being “noise limited.”) The difference between the off-channel signal and the receiver’s noise floor (MDS) is the blocking dynamic range expressed in dB. Again, different combinations of frequencies and signal strengths will produce different blocking behavior, but a higher blocking dynamic range number at 20 kHz indicates better general blocking performance. Typical receivers will have a blocking dynamic range of from 90 to over 150 dB.

Third-order intercept is related, of course, to two-tone, third order IMD. One characteristic of third-order products is that they increase/decrease three times faster than the on-channel products (if the input tones are of equal level). These responses can be plotted on a graph, but the two lines never actually intersect because the receiver always goes into gain compression well before that could happen. So, as signals keep getting stronger, both the on-channel and third-order responses “roll off.” Third-order intercept is the theoretical point at which these two lines would cross. It gives a relative indication of a receiver’s strong signal performance.

If two receivers have the same IMD dynamic range and one has an MDS of –125 dBm and the other –135 dBm, the third-order intercept of the –125 one will be higher. Read that again. Yes, the radio that doesn’t hear as well will give a higher third-order intercept point. That doesn’t mean that you want to always look for a lower third-order intercept point. Third-order intercept should always be evaluated in conjunction with MDS and dynamic range—they are all related.

Q Jeff Lawson, KD1WZ, asks, “Can you help me better understand the nature of the audio/speech circuit in my Kenwood TS-140S transceiver? My concern is that I am not getting full output when operating SSB. The ALC and power meters indicate that the rig is operating properly, but during tests with another station my signal is reported to be almost an S unit stronger when I whistle as opposed to when I am just speaking. Why would there be a difference?”

A The ALC (automatic level control) in your transceiver generates a control voltage to ensure that the peak RF output does not exceed its ratings. It does so by responding to the voltage peaks in the input signal (whether it is human voice or a whistle.) When you’re watching your rig’s ALC meter, you are observing the action of the gain limiting control. The meter is displaying the amount of feedback voltage that is reducing the gain of the transmitter. The ideal situation is to have as much gain as you can without limiting, or with only a tiny bit of limiting. So, the proper way to adjust your mike gain is to set it so that the ALC indicator shows just a little ALC action on your voice peaks.

The ratio between peak and average power in any signal is dependent on the nature of the signal. A whistle is almost a pure sine wave, and its peak and average power are nearly the same. A human voice, however, has a waveform that has a fairly high peak-to-average ratio. Since your radio’s wattmeter indicates full output power for both speech and whistles, it is a peak-reading wattmeter. Many wattmeters read average power, not peak power. Those wattmeters would indicate less power when you are speaking than when you are whistling.

On the receiving end, the S meter is actually measuring the receiver’s AGC (automatic gain control) voltage. A good receiver will generate an AGC voltage from the signal peaks, but this may not always show on the S meter, which may indicate average power, or somewhere in between. If your transmitter’s ALC settings are correct and your transmitter indicates full output power on voice signals, you should rely on that, not on the uncalibrated, unknown performance of a receiver S meter.

A good explanation about power is found in the article “Power—Watt’s It All About?” by Mike Gruber, WA1SVF (now W1MG), May 1995 QST.

Q Is there software available that will allow me to copy ACARS packet transmissions using my PC’s sound card as the analog-to-digital converter?

A For those who may be unfamiliar with the term, ACARS refers to packet data transmissions sent to and from commercial airliners and other ACARS-equipped aircraft. Flight crews use ACARS to report conditions aloft, request information, report minor problems, receive information from their companies and so on.

In the US you’ll hear the 2400-baud AM FSK bursts at 131.550 MHz, with 130.025 and 129.125 MHz as alternate frequencies in busy areas. Interest in ACARS monitoring has increased somewhat in recent years because so many hams own radios capable of receiving in the aeronautical band. ACARS transmissions sound like 1200-baud amateur packet, but at a higher audio pitch. The bursts are also much shorter in duration.

The Doctor knows of at least one freeware ACARS decoder for Windows that utilizes SoundBlaster-compatible sound cards. It requires a Pentium-class PC, but not a speed-demon computer. The software is known as WACARS and you’ll find it on the Web at http://www.mike.mcmail.com/acars.html.

Q We have an idea to promote greater participation in our club activities. We’ll videotape events (such as contest operations, public service events and so on), use a computer “capture card” and software to digitize and edit short video clips, then post the video clips on our club Web site (as Video for Windows AVI files) to show everyone else what they missed. What do you think?

A I think it is a fine idea. In fact, a number of clubs are already doing what you describe. My only suggestion would be to consider another format rather than AVI. As you’ve probably discovered already, AVI files can be huge. Depending on the frame capture rate and other factors, you could be looking at a megabyte per second of video, or even more. A 60-second video clip would be about 60 Mbytes, a big download to say the least!

I’d recommend that you post the clips to your site as MPEG files. MPEG (pronounced EM-pehg), the Moving Picture Experts Group, is actually a committee that evolves standards for digital video and digital audio compression. MPEG-compressed video files are smaller than AVI files, and most of your club members should be able to view them using the Media Player bundled with Windows 95/98, or by using commonly available shareware viewers. (You could even provide a link on your page for members to download a viewer if they don’t already have one.) You don’t mention what video editing software you are considering, but make sure it can generate MPEG files. Some of the less expensive video editing packages do not include this feature.

Q Kaehu Shaprio, WH6WW, asks, “I have about 130 feet of 3/4-inch 75-Ω CATV Hardline that I’d like to use for 2 meters and 70 cm. I looked in The ARRL Antenna Book and found a description of a broadband transformer, but it’s only for 3 to 30 MHz. I also saw an article in the September 1998 QST on how to make a matching transformer, but it seems to only work on one frequency or band. Is there another matching transformer I could build so that I could achieve a 50-Ω match to my transceiver on both of these bands at the same time?”
A I do not recommend that you use 75-to-50 Ω transformers in this application. At 2 meters the loss in your CATV Hardline, if perfectly matched, would be 0.8 dB/100 feet, or a total of 1.02 dB. If you operate this line at a 1.5:1 SWR, the additional loss caused by the SWR would be 0.07 dB. It is very unlikely that you could obtain less than 0.07 dB of total loss between two matching transformers—one on each end. Instead of building transformers, why not simply use the Hardline as it is? The SWR on the line will be approximately 1.5:1 and the loss, even at 70 cm, will be negligible. Most likely, your transmitter will be perfectly happy to deliver full power into that load.

Q John Duncan, WA5ZVE, asks, “I currently have a 52 foot crank-up tower that is about 5 feet from the back of my house. Additionally, I live on a lot that is about 65 × 110 feet. I am looking at replacing my existing tower with a 72-foot model. I also want to make sure that both my current and future installations are in compliance with the new RF safety regulations. When I read the RF-exposure regulations, I noticed that they require hams to know their peak power, but then they talk about average power and average exposure. I also noticed that there is a 500-W limit on some bands. This appears to mean that I can’t run my 1500-W amplifier with either installation and still be in compliance. Did I miss something?”

A Determining RF safety compliance can be confusing, which is why we published the ARRL RF Exposure Book. (Please excuse the shameless plug!)

Acutally, there are two power levels that you need to consider. The first is the peak output power to your antenna. This level determines if you need to do a station evaluation. On 160, 80 and 40 meters those stations that run 500 W PEP or less do not have to be evaluated. On 30 meters, the level is 425 W. On 20 meters the level is 225 W, on 15 it is 100 W and on 10 it is 50 W. This doesn’t mean that you can’t run more power, but greater output would require an evaluation.

When you do your evaluation, you can use your average power. To calculate this, start with PEP. Multiply that by the duty factor of the mode you are using: 20% for SSB with no processor, 40% for CW or heavily processed SSB, 100% for RTTY or FM. Multiply that result by the percentage of time you might be transmitting during the averaging period. Let’s talk uncontrolled/general public exposure, so we will use 30 minutes. As an example, if you are a high-power conversational CW operator, you should probably use 400 W (1500 W × 40% × 2/3 [20 minutes on out of 30]). From that level, you can do your evaluation.

There are a lot of ways you can do an evaluation. The FCC published Supplement B that has a number of tables. These tables show you how far you need to be from your antenna to comply. For example, on 10 meters, the HF band with the most stringent requirements, if you are running 500 W average power to a typical 3-element Yagi, your neighbors must be 54 feet away from your antenna, diagonally. In your case, they would be, so the dreaded evaluation is over and you just passed! You may have other antennas to analyze but, as an example, again using the simple FCC tables, 500 W average power to a 40-meter dipole requires 6.9 feet separation between the antenna and neighbors. That’s an easy “pass,” too.

It looks like you can use your amp with either the 50 or 70 foot tower. What you probably can’t do, at least with the shorter tower, is transmit a 30-minute continuous carrier at full output on 10 meters. That would require that your neighbors be 95 feet from the antenna, so you can only do it when your neighbors are not standing for 30 minutes on the property line. Of course, there is also the issue of potential damage to your amplifier, your reputation on the air and so forth!

Q James Pirkle, KR4QN, asks, “The National Weather Service has recently started transmitting SAME in the Atlanta area. How can I learn more about this technology?”

A SAME—Specific Area Message Encoding—allows the National Weather Service to broadcast warnings and other weather information for specific counties. You simply program a SAME-compatible radio with the code for your county. Once the radio is programmed, it will remain silent until it hears a bulletin specifically intended for your area.

Of course, the trick is knowing which SAME code to program in your receiver. Fortunately, the National Weather Service has made it easy to get this information on-line. They operate an excellent Web site at http://www.nws.noaa.gov/nwr/ and they include a table of SAME codes for all counties in the US where the transmissions can be currently received. According to the NWS site, the SAME code for your county (DeKalb) is 013089 and the NWS station is KEC80 on 162.550 MHz.

If you listen to the actual tones that go out on the air when a weather alert is transmitted, you will hear the familiar long tone first, followed by some short bursts of data that sound like packet information. The first tone is used to activate the “conventional” weather alert radios and the packet-like bursts are the SAME warnings.

At present there is only one manufacturer producing consumer grade weather radios with the SAME decoder in them, and that is Radio Shack. No doubt other manufacturers will jump on the bandwagon soon.

Q Craig Cochran, N5KYF, asks, “I have a receiver that I like to power with NiCd batteries. But since the radio can operate over a very wide range of supply voltages, the batteries are exhausted before the radio starts to sound weak. (It takes six ‘D’ cells but seems to work okay down to 4 or 5 V.) “So, I need to build a circuit that will turn the radio off (disconnect the batteries)when battery voltage drops below about 6 V. I also need a circuit that will not draw much current (the whole idea is to save the batteries!). Could I use a 6-V Zener diode and a switching transistor?”

A If you were going to make a circuit that switched off your radio, the energy to measure the voltage and perform the switching function would come from where? The battery! It is an ironic fact that any kind of battery indicator ultimately runs the battery down faster than if it weren’t there at all. Users of H-Ts who like the security of a built-in battery checker beware! I’m not saying the amount of current drain is significant, but it’s there nonetheless.

I think a better approach is to use an indicator that draws very little current and leaves the act of shutting off the radio to you. That way you can at least have a little control over the process. The lowest current-drawing indicator that I can think of is a single segment of an LCD display. Although it would draw current itself in the act of monitoring the battery voltage, the amount should be miniscule. Just think of how long the LCD watch face on your wristwatch runs off that tiny cell inside. In the October QST there is a also nifty little circuit for monitoring the condition of your battery. It draws very little current. See “A Battery-Voltage Indicator” by Donald G. Varner, WB3CEH, on page 50.

How about an extreme “low tech” approach? It could be something as simple as putting a subminiature momentary contact switch in series with the battery so that it only works for the split second when you press the button. A tiny meter movement could take the reading for you. You could accurately measure the battery voltage and it wouldn’t be drawing anything except during the moment you pressed the button. Even then the current drain would be insignificant.

Q Charlie Fortner, KF4GJQ, asks, “I’ve just bought a new digital multimeter (DMM), but I notice that it doesn’t measure capacitance or inductance. Is there any way to measure these quantities with a standard DMM?”

A A DMM cannot tell you the inductance of a coil, which is the coil’s most important characteristic. If you attempt to measure the coil’s resistance, you’re likely to discover that it is so low that your DMM will read a dead short (zero ohms or something close to it).

By the same token, a DMM cannot divine capacitance directly. If you attempt to measure the resistance of a nonpolarized capacitor, you may see a very quick “bump” in the meter reading as the capacitor charges up to the voltage available across the meter’s probes. The DMM should very quickly indicate close to infinite resistance for a good nonpolarized capacitor.

In the case of a polarized capacitor such as an electrolytic, you will probably see more pronounced and prolonged charging activity when you first put the probes across the capacitor (evidenced by a low-resistance reading while the cap charges). The resistance will gradually rise to several hundred thousand ohms when fully charged.

The best approach may be to build outboard adapters that will allow your DMM to measure inductance or capacitance. See page 26.22 in either the 1997 or 1998 editions of The ARRL Handbook. If you want a commercial device that’s specifically designed to measure inductance and capacitance, you’ll need to purchase an LCR meter. LCR meters offer a fair degree of accuracy, but good ones will set you back $200 to $300. That makes the home-brew Handbook adapters look pretty attractive!

Q Richard Bauer, K5RB, asks, “Do you have any cures for HF interference caused by the computer in my shack?”

A Computers are notorious RF generators. Even computers with grounded metal cases can leak RF by various routes. The first thing to do is to narrow down the list of suspects. Turn on your computer system and your radio. Listen to an interference signal, turn off your PC, unplug a peripheral cable, and turn the PC back on. (Peripheral cables include those to your monitor, printer, scanner, mouse and even your keyboard.) If the interference suddenly disappears or drops sharply, investigate either the cable you just disconnected or the device it is attached to. A ferrite choke on the offending cable may help. Or, relocate the “leaky” device.

If you’ve disconnected all the cables but the interference remains, suspect either the computer’s switching power supply or RF coming from bus cards or the motherboard itself. Fixing a noisy power supply would entail installing a device that replaces the existing female ac socket with one that has a built-in filter. You may find these in computer-supply stores. This is done to try to prevent the RF generated in the power supply from getting to the ac line and using the cord as an antenna.

But if the noise is coming from the internal circuitry, you have few options left. Some hams have gone as far as covering the outsides of their PC cabinets with grounded copper mesh, but this is a bit extreme! The other option is to move your antenna if possible. Most computer interference is received at the antenna, so relocating the antenna may help.

Take a look at Chapter 18 in The ARRL Handbook, or pick up a copy of The ARRL RFI Book for more suggestions.